In an optical scanner used in a laser printer or the like in the background art, a polygon mirror and a photoconductor drum are generally used as an optical deflector and a to-be-scanned surface respectively. A scanning optics is constituted by a lens, a mirror or a combination of them. With increase in speed and density in recording, there is often the case where a plurality of scanning lines are formed concurrently by use of a plurality of beams of light.
To use a plurality of beams, light sources emitting the beams respectively are disposed at an interval of a small distance in a direction (hereinafter referred to as “sub-scanning direction”) perpendicular to a direction (hereinafter referred to as “main scanning direction”) in which the beams are deflected and scanning is performed therewith. When the small distance is multiplied by an optical magnification between the light sources and the photoconductor drum surface, a predetermined interval is obtained. The beams form scanning lines at the predetermined interval on a photoconductor drum surface.
In a range (hereinafter referred to as “scanning range”) to be scanned with the deflected beams, the optical magnification may be uneven. In this case, the interval of the plurality of scanning lines on the photoconductor drum surface will be uneven in the scanning range. Particularly when there is a finite angle between the beams incident on the polygon mirror and the optical axis of the scanning optics, the optical path length will change asymmetrically with respect to the optical axis of the scanning optics with the rotation of the polygon mirror. Thus, the aforementioned optical magnification will be generally asymmetric in the scanning range.
For example, there are methods for making the optical magnification even, in which the scanning optics is made of a lens, using two surfaces whose sub-scanning-direction curvatures are changed continuously in the main scanning direction and independently of the main-scanning-direction curvatures respectively (see undermentioned Patent Documents 1 to 3).
These use a system in which the power distributions of the two surfaces are optimized, that is, the position of a composite principal point is optimized, so that the ratio of the distance between the principal point and an object point to the distance between the principal point and an image point can be fixed independently of the scanning angle. Here, the more closely the two surfaces are disposed, the largely the power of each surface has to be changed to move the position of the composite principal point. In this specification, the power means an operation to bend light, such as refraction, convergence, etc.
For example, Patent Document 1 discloses an example in which one of the surfaces has a negative power (divergence of light) while the other surface has a positive power (convergence of light). The system is apt to be affected by an error due to large absolute values of the two powers.
In order to avoid this, it is desired to dispose the two surfaces as separately as possible in the optical-axis direction. In that case, the surface disposed on the side of the to-be-scanned surface has a comparatively large size in the main scanning direction, that is, longitudinally.
In order to provide the scanning optics with an operation for correcting an optical face tangle error of the polygon mirror, the reflection plane of the polygon mirror and the to-be-scanned surface are generally disposed to be conjugated. In this event, the system is affected by the power of each surface and a location error more easily as the optical magnification between the reflection surface of the polygon mirror and the to-be-scanned surface increases. Also for this reason, it is desired to dispose the composite principal point as closely to the to-be-scanned surface as possible. To this end, the surface disposed on the to-be-scanned surface side has a comparatively large longitudinal size.
A lens having a varying curvature and a comparatively large longitudinal size as described above is generally formed by plastic molding. There is a tendency that it is more difficult to secure the shape accuracy and the homogeneity on a high level as the longitudinal size increases.
There is another method for making the optical magnification even in the scanning range. For example, two surfaces whose sub-scanning-direction curvatures are changed continuously in the main scanning direction and independently of the main-scanning-direction curvatures respectively are used, while those changes are made asymmetrical with respect to the main scanning direction (see undermentioned Patent Document 4). This is an excellent system in which an asymmetrical change of the optical path length can be corrected to make the optical magnification symmetric and even. Due to the plurality of asymmetrical surfaces, it is, however, difficult to align the axes of the surfaces.
There is further another method for making the optical magnification even in the scanning range. For example, a plurality of reflection surfaces having powers in the main scanning direction and the sub-scanning direction and having no rotationally symmetrical axis are used (see undermentioned Patent Document 5). Due to the plurality of rotationally symmetric reflection surfaces having powers both in the main scanning direction and the sub-scanning direction, the accuracy in aligning the axes of the surfaces becomes severe.
There is another disadvantage that the reflection surfaces are greatly affected by a curvature radius error and shape accuracy in comparison with refraction surfaces. Particularly in plastic molding of a reflecting mirror having a longitudinal size, it is difficult to secure high accuracy also due to the influence of expansion and deformation caused by a change in environmental temperature.
Further, when the reflection surfaces are produced by cutting an aluminum material or the like, the integral of a stress vector applied from a to-be-cut piece to a tool serving as a cutter has an angle with respect to the central axis of the tool. That is, force is applied to the tool unequally. That is therefore disadvantageous to the life of the tool and the stability of the accuracy.
The influence of this inequality rarely causes any problem in molding a small number of products. However, when individual reflection surfaces are processed repeatedly, the influence is considerable in terms of an accuracy variation. As for reflection surfaces of an aluminum material about 300 mm in longitudinal size, it is estimated that 50-100 can be processed with one diamond tool.
In a grindstone cutting method in which a cutting tool has a comparatively long life, there is a limit in the size which can be processed by a cutting device. Particularly due to clogging of a grindstone, the method cannot be applied to an aluminum material. Further, the amount of cutting is proportional to the square of the size. Because the cutting quantity is proportional to the square of the size, the influence when the reflection surfaces have power in the main scanning direction, that is, longitudinally becomes very large in terms of the life of the tool and the stability of the accuracy as compared with that when the reflection surfaces have power in the sub-scanning direction.
Patent Document 1: JP-A-9-33850
Patent Document 2: JP-A-2000-121985
Patent Document 3: JP-A-2001-4951
Patent Document 4: JP-A-2001-194611
Patent Document 5: JP-A-2000-275557